I know that if you have a loop that modifies the count of the items in the loop, using the NSEnumerator on a set is the best way to make sure your code blows up, however I would like to understand the performance tradeoffs between the NSEnumerator class and just an old school for loop
Using the new for (... in ...) syntax in Objective-C 2.0 is generally the fastest way to iterate over a collection because it can maintain a buffer on the stack and get batches of items into it.
Using NSEnumerator is generally the slowest way because it often copies the collection being iterated; for immutable collections this can be cheap (equivalent to -retain) but for mutable collections it can cause an immutable copy to be created.
Doing your own iteration — for example, using -[NSArray objectAtIndex:] — will generally fall somewhere in between because while you won't have the potential copying overhead, you also won't be getting batches of objects from the underlying collection.
(PS - This question should be tagged as Objective-C, not C, since NSEnumerator is a Cocoa class and the new for (... in ...) syntax is specific to Objective-C.)